Using an optical head to access data on an optical recording medium is a technique known in the art. While the storage capacity and density of the optical recording medium has increased gradually, the structure of the optical head has also improved. One of the advanced features is that the wavelength of laser light to access the optical recording medium becomes shorter, and the numerical aperture (NA) of the objective lens gradually increases (i.e. the converging focal point of the laser light to access the optical recording medium becomes smaller, its size is in direct proportion with the wavelength of the laser light, but is in inverse proportion with the NA of the objective lens) to respond to the increasing storage capacity and density of the optical recording medium.
The new type of optical head has to be inversely compatible. Namely, it must be able to access new types of optical recording media and also has to access the older types of optical recording media. Hence the new type of optical head has to equip with a read/write mechanism for laser light of different wavelengths. As a result, optical recording media capable of storing data of two different densities have been developed. For instance, now many optical heads can read/write a Compact Disk (CD) and a Digital Versatile Disk (DVD).
One example is U.S. Pat. No. 5,446,565, which discloses a dual focal points converging objective lens that can form different NA of the objective lens. When a laser light generation unit generates laser light, which travels to a holographic optical element (HOE), the laser light forms diffraction because of the HOE, and the converging objective lens focuses the light on an optical recording medium. Using the characteristics of the HOE that can form two diffraction angles, and is coupled with the converging objective lens, the laser light may be converged to different foal points (the data surface of the optical recording medium that has different data storage densities) to access the optical recording medium that has two different data storage densities. However, the HOE is more expensive in fabrication.
Based on cost consideration, employing two light sources and two objective lenses is a more economic choice. Korea patent No. 00255233 discloses a technique that uses laser light of two different wavelengths to couple with different lenses to provide different focal points and read optical spots of different diameters. The optical head allows laser lights of different wavelengths generated by two laser light generation units to travel their own optical paths to reach the converging objective lens. And according to different NA of the objective lens corresponding to the laser light of different wavelengths, different sizes of focal points are formed. After reflected by the optical recording medium, each travels back to its own photo detector, thereby can access the optical recording medium that has two different data storage densities. But the dual optical paths system significantly increases the size of the whole device. This is against the prevailing requirements of 3C products.